The same brothers who collected fairy tales found a law hiding in words. Latin and English share ancestors, and one whole class of consonants shifted on the way to English — every p became f, every t became ‘th’, every k an h. Latin pater is English father. Not coincidence: a rule. Slide through the cognates and watch it fire.
Grimm’s Law (1822) describes the First Germanic Sound Shift: the Proto-Indo-European voiceless stops p, t, k became the fricatives f, þ (‘th’), h in Germanic, while voiced stops b, d, g devoiced to p, t, k. It is a SYSTEMATIC, exceptionless correspondence (Latin, which kept the old stops, lines up against English, which shifted): pater/father, trēs/three, cornu/horn, duo/two. A fail-loud self-check throws unless the shift table maps p→f, t→th, k→h and reproduces the cognate pairs — a law you can run, not a list to memorise.
Shown as a clean letter-substitution over selected cognates; the real shift is over PHONEMES with conditioning environments (Verner’s Law explains the ‘exceptions’), and spelling hides some of it. The regular p→f / t→th / k→h correspondence is exact.